The focus of research will shift from the investigation of normal to neoplastic, transformed B lymphocytes. We have found that the I 29 tumor consists of a system of B cells with a limited potential for differentiation. The cells express Ia and two immunoglobulin isotypes on their surface, and can be induced to secrete one of them, IgA. We will investigate this tumor to learn about the direction of differentiation, the association of Ig changes with changes in cell surface antigens, the regulation of I 29 differentiation by factors or tissues in the mouse as well as in culture. Furthermore, we hope to clarify some aspects of regulation of Ig expression in the population of I 29 expressing both isotypes at once. Our efforts to generate and to characterize monoclonal antibodies to B cell antigens will continue. Antibodies to Ia, PC.2, and Ly 6 available in our laboratory will be analyzed in more detail. The relationship of expression of the above antigens with B cell development will be studied.